Microsoft has announced the upcoming end of mainstream support for SQL Server 2005 SP2 on January 12, 2010, and for SQL Server 2008 RTM on April 13, 2010. This just means that you need to upgrade to a newer Service Pack by then (which you should do anyway, IMHO).
They also announced that there will be fixes for a number of security issues in SQL Server 2005 SP2 and SP3 that they rate as Critical in this month’s “Patch Tuesday” coming on October 13, 2009. As far as I can tell, SQL Server 2008 is not affected by these issues. My test and production database servers are all running SQL Server 2008 SP1 or better, so I don’t have worry about either of these items.